CompSci Today’s topics Java Writing Functions/Methods Upcoming Information Retrieval Reading Great Ideas, Chapter 4

CompSci Writing Functions/Methods  Function is Synonym of Method  Function is more generic term  Method is used in Java  Syntax of a function return-type name(parameter-list) { statement;... statement; return return-type-object // if return-type not void }  Return-type  May be void if no information returned  If not void, type must match type of info returned  If not void, 1 or more return statements required

CompSci Writing Functions/Methods  Parameter list  Type-object pairs, separated by commas  Can be empty  Parameters used to obtain info from outside of function  Return statement  Sends information back to statement that invoked function  Type must match return-type on function header  Miscellaneous  Header may be preceded by public to make function available anywhere in program  Can make private to insure “internal use only”

CompSci  Void Functions  Use by putting on line with parameters (if any)  E.g., object-name.setText(”Hello”);  Functions that return info  Must use in calculation or assignment  Include parameters (if any)  E.g., weight = object-name.getDouble();  What does the following do? Object-name.getInt(); Use of Functions

CompSci Simple Uses of a Simple Function // definition: public double sumTres(double a, double b, double c) { double sum; sum = a + b + c; return sum; } // uses: double measure = sumTres(14.3, 12.1, -4.2); m2.setText("Alltogether we have: " + measure); // or: double x = d1.getDouble(); double total = sumTres(27.9, x, d2.getDouble()); m2.setText("It all adds up to " + total); // or: m2.setText("Sum: " + sumTres(-3.2, d3.getDouble(), 1.0));

CompSci Writing Functions public class DiaFuncts extends java.applet.Applet implements ActionListener { TextField tf; TextArea ta; Button bDraw; String stars = "*******************"; String spaces = " "; public void init() { tf = new TextField("Hello "); ta = new TextArea(20, 20); ta.setFont(new Font("Monospaced", Font.BOLD, 12)); bDraw = new Button("Draw"); bDraw.addActionListener(this); add(tf); add(bDraw); add(ta); }

CompSci Writing Functions.2 void Dia() { int k = 0; while (k < 10) { ta.append(spaces.substring(0,10-k) + stars.substring(0,2*k+1)+"\n"); k = k + 1; } void Mond() { int k = 1; while (k < 10) { ta.append(spaces.substring(0,1+k) + stars.substring(0,19-2*k)+"\n"); k = k + 1; }

CompSci Writing Functions.3 public void actionPerformed(ActionEvent event){ Object cause = event.getSource(); if (cause == bDraw) { tf.setText("Goodbye"); Dia(); Mond(); }  No New Capability  Just packaged differently from Diamond.java code

CompSci Using Parameters  Now take advantage of parameters to set size of diamond public class DiaParms extends java.applet.Applet implements ActionListener { IntField gSize; TextArea ta; Button bDraw; String stars = "*******************"; String spaces = " "; public void init() { gSize = new IntField(10); ta = new TextArea(20, 20); ta.setFont(new Font("Monospaced", Font.BOLD, 12)); bDraw = new Button("Draw"); bDraw.addActionListener(this); add(gSize); add(bDraw); add(ta); }

CompSci Using Parameters.2 void Dia(int sz){ // this line changed int k = 0; while (k < sz){ // this line changed ta.append(spaces.substring(0,10-k) + stars.substring(0,2*k+1)+"\n"); k = k + 1; } void Mond(int sz){ // this line changed int k = sz; // this line changed while (k > 0) { k = k - 1; ta.append(spaces.substring(0,10-k) + stars.substring(0,2*k+1)+"\n"); }

CompSci Using Parameters.3 public void actionPerformed(ActionEvent event){ int sz; // this line new Object cause = event.getSource(); if (cause == bDraw) { sz = gSize.getInt(); // this line new Dia(sz); // this line changed Mond(sz-1); // this line changed }  Almost no difference in code for functions  Great difference in flexibility

CompSci The Partially Used Array  Often array is sized for worst case  Only use the “front” part of it in most cases  Need variable to keep track of how much we use o Such variable often called size or used  Saw an example in ArrayStats program: double mean(double[] list, int size) { int k = 0; double sum = 0.0; while (k < size) { sum = sum + list[k]; k = k + 1; } return sum/size; }

CompSci Functions that work on Arrays  The previous was an example of using an array parameter  Such functions that “work” on arrays are common  Possible Applications:  Sum values double arraySum(double data[], int size);  Count number of occurrences of a specific value int arrayCountZero(double data[], int size); int arrayCount(double data[], double val, int size)  Set all values to a constant value void arraySet(double data[], double val, int size);  Locate (search for) a particular value int arrayFind(double data[], double key, int size);

CompSci Information Retrieval  Often want to use program for information storage and retrieval  On-line phone book is good example  Using “Parallel” or “Corresponding Arrays”  Array (of Strings) for Names  Array (of Strings) for Phone Numbers Name Number “J.Able” “D.Ramm” “R.Odom” “M.Salter” “W.Tars” “ ” “ ” “ ” “ ” “ ” “ ”